Liquid crystal device operable in two modes and method of operating the same

Abstract

A liquid crystal device is provided that comprises a layer of liquid crystal material ( 2 ) disposed between two cell walls ( 4, 6 ) in an arrangement such that the liquid crystal material ( 2 ) can adopt any one of two or more stable liquid crystal configurations that will persist in the absence of an applied electric field. The liquid crystal device is operable in two modes; a first mode in which application of an appropriate latching voltage pulse can select any one of the two or more stable liquid crystal configurations, and a second mode in which application of an electric field can switch the layer of liquid crystal material from a latched configuration to a switched configuration and in which the layer of liquid crystal material will return to said latched configuration when the applied electric field is removed.

1. A liquid crystal device comprising, a layer of liquid crystal material disposed between two cell walls in an arrangement such that the liquid crystal material can adopt any one of two or more stable liquid crystal configurations that will persist in the absence of an applied electric field, the liquid crystal device being operable in two modes; a first mode in which application of an appropriate latching voltage pulse can select any one of the two or more stable liquid crystal configurations, and a second mode in which application of an electric field can switch the layer of liquid crystal material from a latched configuration to a switched configuration and in which the layer of liquid crystal material will return to said latched configuration when the applied electric field is removed. 
     
     
       2. A liquid crystal device according to  claim 1  wherein the device is latched in to a particular stable configuration prior to operation in the second mode. 
     
     
       3. A liquid crystal device as claimed in  claim 1  wherein the arrangement is such that the liquid crystal material can adopt any one of three or more stable liquid crystal configurations. 
     
     
       4. A liquid crystal device as claimed in  claim 1  wherein the arrangement is such that the liquid crystal material can adopt any one of two stable liquid crystal configurations. 
     
     
       5. A liquid crystal device as claimed in  claim 4 , wherein a surface alignment grating is provided on the internal surface of at least one cell wall that is adapted to provide two stable liquid crystal configurations. 
     
     
       6. A liquid crystal device as claimed in  claim 5  wherein the liquid crystal material comprises nematic liquid crystal material. 
     
     
       7. A liquid crystal device as claimed in  claim 5  that comprises a surface alignment grating provided on the internal surface of the first cell wall and a planar surface treatment provided on the internal surface of the second cell wall, the arrangement being such that one of the stable liquid crystal configurations is a twisted nematic configuration. 
     
     
       8. A liquid crystal device, as claimed in  claim 7  in which the twist of the liquid crystal material in the twisted nematic configuration is greater than 45°. 
     
     
       9. A liquid crystal device as claimed in  claim 7  in which the twist of the liquid crystal material in the twisted nematic configuration is greater than 90°. 
     
     
       10. A liquid crystal device as claimed in  claim 7  wherein the device is latched into the twisted nematic configuration prior to operation in the second mode. 
     
     
       11. liquid crystal device as claimed in  claim 5  that comprises a surface alignment grating provided on the internal surface of the first cell wall and a homeotropic surface treatment provided on the internal surface of surface of the second cell wall, the arrangement being such that one of the stable liquid crystal configurations is a vertically aligned nematic configuration. 
     
     
       12. A liquid crystal device as claimed in  claim 11  wherein the device is latched into the vertically aligned nematic configuration prior to operation in the second mode. 
     
     
       13. A liquid crystal device as claimed in  claim 1  wherein the liquid crystal material comprises a nematic (or long pitch cholesteric) material. 
     
     
       14. A liquid crystal device as claimed in  claim 13  wherein the liquid crystal material further comprises cholesteric liquid crystal material. 
     
     
       15. A liquid crystal device as claimed in  claim 14 , wherein the liquid crystal material comprises less than 0.02% by weight of cholesteric liquid crystal material. 
     
     
       16. A liquid crystal device as claimed in  claim 14  wherein the ratio of liquid crystal layer thickness to the pitch of liquid crystal material is within the range of 0.25 to 1.25 inclusive. 
     
     
       17. A liquid crystal device as claimed in  claim 1  wherein the liquid crystal material has a positive dielectric anisotropy. 
     
     
       18. A liquid crystal device as claimed in  claim 1  wherein the liquid crystal material has a negative dielectric anisotropy. 
     
     
       19. A liquid crystal device as claimed in  claim 1  wherein the cell walls comprise electrodes for applying an electric field to the liquid crystal, the electrodes being arranged to form a matrix of addressable pixels. 
     
     
       20. A liquid crystal device as claimed in  claim 19  that comprises row electrodes formed on the first cell wall and column electrodes formed on the second cell wall, the first and second cell walls being arranged so as to define an array of passively addressable pixels. 
     
     
       21. A liquid crystal device as claimed in  claim 19  that comprises a means of independently applying an electric field to each individual pixel. 
     
     
       22. A liquid crystal device as claimed in  claim 21  wherein the means of independently applying an electric field to each individual pixel comprises a thin film transistor element. 
     
     
       23. A liquid crystal device as claimed in  claim 22  wherein a storage capacitor is associated with each thin film transistor element. 
     
     
       24. A liquid crystal device as claimed in  claim 19  which includes at least one discrete semiconductor circuit capable of addressing two or more pixels. 
     
     
       25. A liquid crystal device as claimed in  claim 19  wherein the device is adapted such that the first mode of operation and the second mode of operation are capable of simultaneous use on different pixels or groups of pixels. 
     
     
       26. A liquid crystal device as claimed in  claim 19  wherein pixels that are to be operated in the second mode are blanked into a predetermined stable liquid crystal configuration prior to operation in the second mode. 
     
     
       27. A liquid crystal device as claimed in  claim 1  wherein the driving voltages applied to the device are substantially d. c. balanced over time. 
     
     
       28. A liquid crystal device as claimed in  claim 1  wherein the second mode is adapted to display images at video rate. 
     
     
       29. A liquid crystal device as claimed in  claim 1  wherein the second mode is adapted to display images with 256 grey-levels. 
     
     
       30. A liquid crystal device as claimed in  claim 1  comprising at least one polariser to distinguish between different liquid crystal configurations. 
     
     
       31. A liquid crystal device as claimed in  claim 1  wherein the device further comprises a reflective means and the device is configured to operate in reflective mode. 
     
     
       32. A liquid crystal device as claimed in  claim 1  and additionally comprising colour filter elements. 
     
     
       33. A liquid crystal device as claimed in  claim 1  and further comprising electronic means for selecting first mode or second mode operation. 
     
     
       34. An information display apparatus comprising a microprocessor unit adapted to electronically address a liquid crystal device according to  claim 1 . 
     
     
       35. A method of operating a liquid crystal device comprising the steps of; taking a liquid crystal device that is arranged such that the liquid crystal material can adopt any one of two or more stable liquid crystal configurations that will persist in the absence of an applied electric field, and determining whether to operate the device in a first mode or a second mode, whereby in the first mode of operation any one of the two or more stable liquid crystal configurations can be selected by application of an appropriate latching voltage, and in the second mode of operation the application of an electric field can switch the layer of liquid crystal material from a latched configuration to a switched configuration and in which the layer of liquid crystal material will return to said latched configuration when the applied electric field is removed. 
     
     
       36. A method as claimed in  claim 35  in which the liquid crystal device comprises a plurality of pixels and in which only some of the pixels are operated in the second mode. 
     
     
       37. A method of operating a device as claimed in  claim 35  whereby the liquid crystal material is latched in to a particular stable configuration prior to operation in the second mode. 
     
     
       38. A method of operating a zenithal bistable device as claimed in  claim 35 .